1. Field of the Invention
The present invention relates to a solvated electron lithium electrode comprising a solution of lithium dissolved in liquid ammonia contained by a lithium intercalating membrane. The solvated electron lithium electrode is suitable for use in a rechargeable high energy density cell or battery utilizing a liquid lithium ion conducting non-aqueous electrolyte and a solid positive electrode, or utilizing liquid lithium ion conducting positive electroactive material.
2. Description of the Prior Art
Secondary cells utilizing essentially pure lithium electrodes as anodes with lithium ion conducting non-aqueous electrolytes generally exhibit less than Faradaic cycling efficiency. Lithium electrodes are prone to undergo surface morphological changes during electrochemical cycling which lower the overall coulombic efficiency of the cell. The reduction in coulombic efficiency represents an irreversible loss in lithium capacity after each cell cycle. During cell charging, electrodeposited lithium reacts with the non-aqueous electrolyte to form an insulating film at the lithium electrode/electrolyte interface. This electrochemically deposited lithium film is non-uniform and dendritic areas develop which become electrically isolated from the lithium negative active material. During subsequent discharge, lithium particles become susceptible to mechanical removal from the electrode without contributing to the overall Faradaic charge capacity of the electrode. Lithium particles lost in this manner are generally unavailable for further cell cycling. This type of irreversible lithium loss due to morphological changes at the lithium electrode/non-aqueous electrolyte interface region occurs when unit activity lithium is deposited during electrode charge.
Electrodes consisting of intercalation compounds for high energy density batteries, and the importance of intercalation electrodes in solid state chemistry is known. See, e.g., M. B. Armand, "Intercalation Electrodes", Material for Advanced Batteries, D. W. Murphy, J. Broadhead, eds., Nato Conference Series VI, p. 145 (1979). Intercalation compounds undergo topochemical reactions involving the insertion of a guest into the intercalation compound host lattice structure with minimal structural changes by topotactic electron/ion transfer. Intercalation reactions are generally completely reversible at ambient temperatures and pressures, and therefore utilization of intercalation compounds in secondary cells is very promising.
Lithiated rutiles have been utilized as electrodes in rechargeable electrochemical cells. The topochemical lithiation of rutile related structures in non-aqueous lithium electrochemical cells is taught in D. W. Murphy et al, "Topochemical Reactions of Rutile Related Structures with Lithium", Mat. Res. Bull. Vol. 13, 1395 (1978). This article relates to the use of transition metal chalcogenides, oxides and oxyhalides as host structures suitable for use as cathodes in room temperature batteries utilizing lithium as the guest. Rutile related metal dioxides, in particular, exhibit a range of important parameters for lithium incorporation which suggest their suitability in high energy density battery applications, such as the range of size and vacancy for lithium, diffusion pathway, electronic conductivity, and crystallographic distortion.
One researcher suggests that intercalation of lithium ions may be achieved by reaction of the host lattice with a lithium/ammonia solution to provide an intercalated solid electrode. R. Schollhorn, "Reversible Topotactic Redox Reactions of Solids by Electron/Ion Transfer", Angew. Chem. Int. Ed. Engl. 19: 983 (1980). This article also teaches that much experimental work has been conducted with Li/TiS.sub.2 cells having a solid lithium anode and TiS.sub.2 layered dichalcogenide cathode. The role of ternary phases in lithium anodes and cathodes comprising metallic halide, oxide and chalcogenide intercalation compounds is elucidated in M.S. Whittingham, "The Role of Ternary Phases in Cathode Reactions", J. Electrochem. Soc.; 123: 315 (1976).
Cells have also been proposed having two intercalation electrodes, each intercalation electrode having a different lithium activity. M. Lazzari and B. Scrosati, "A Cyclable Lithium Organic Electrolyte Cell Based on Two Intercalation Electrodes", J. Electrochem. Soc.; 127:773 (1980).
Solutions of alkali and alkaline earth metals in liquid ammonia are known to exhibit high ionic and electronic conductivity, and utilization of such materials in galvanic cells has been proposed. Dilute solutions of lithium or sodium in ammonia have a characteristic deep blue color, and the solution takes on a bronze or metallic appearance at greater concentrations. According to the solvated electron model, an electron is removed from the alkali or alkaline earth metal and resides in cavities created by the association of several ammonia molecules. The ammoniated electrons are considered to be associated with molecular orbitals located on the ammonia protons.
In general, the technical obstacle to the application of electroactive solvated electron solutions in rechargeable galvanic cells has been the difficulty in providing appropriate containment of the solvated electron solution. Separation of the solvated electron solutions from positive electroactive materials and electrolyte while maintaining low internal resistance is important in high energy density battery applications.
Ambient temperature secondary batteries using a solvated electron electrode comprising sodium or sulfur dissolved in liquid ammonia have been developed. J. Bennett et al, "The Solvated Electron Battery", 18th IECEC Meeting 1665 (1983). Secondary cells containing sodium solvated electron solutions utilizing a sodium ion conducting solid electrolyte, such as .beta."-alumina electrolyte, have unacceptably low ionic conductivity at ambient temperatures, and require high operating temperatures. Cells were also developed using a solvated electron sulfur electrode with a sulfinated styrene separator which demonstrated poor containment.
Studies relating to the self-decomposition reaction of concentrated solutions of lithium and ammonia at atmospheric pressure are reported in M. H. Miles and W. S. Harris, "Decomposition Reaction of Concentrated Lithium-Ammonia Solutions", J.Electrochem. Soc., 121: 459 (1974). This publication suggests that solutions of lithium in liquid ammonia could provide an interesting electrochemical fuel for fuel cells or batteries. In the absence of an enclosed vessel, lithium/ammonia solutions slowly decompose by the reaction: EQU Li(NH.sub.3).sub.x .fwdarw.LiNH.sub.2 +(x-1)NH.sub.3 (g)+1/2H.sub.2 (g)
as a consequence of the continuous removal of gaseous ammonia and hydrogen. In an enclosed (pressurized) environment, however, this reaction is reversible and decomposition is arrested.